1. Field of the Invention
The invention relates to a device for the inductive heating of a sleeve section of a toolholder which contains a central holding opening for a shaft of a rotary tool, for example a drill or mill or reaming tool, said toolholder holding the shaft of the tool seated in the holding opening with a press fit and releasing it when heated.
2. Background of the Related Art
It is known, in particular in the case of tools rotating at high speed, to shrink their shaft into a sleeve section of a toolholder. For this purpose, the sleeve section is heated, for example by means of an induction coil which encloses it, so that the tool shaft can be plugged into the holding opening in the sleeve section, which is therefore enlarged. The external diameter of the tool shaft is somewhat greater than the nominal diameter of the holding opening, so that, after the sleeve section has cooled down, the tool is held firmly by a press fit in the toolholder so as to rotate with it.
In order to remove the tool, the sleeve section has to be heated again. Since, in this case, there is the risk that the tool shaft will also be heated, there can be problems if the thermal expansion of the sleeve section is inadequate, as based on the tool shaft, which likewise expands.
DE 199 15 412 A1 discloses an inductive heating device for heating the sleeve section of a toolholder. The device has an induction coil which can be placed on the sleeve section of the toolholder and, in this case, encloses the latter annularly with a radial spacing and which is fed with alternating electric current from a generator. The magnetic field of the induction coil induces induction currents in the electrically conductive, generally also magnetizable, material of the toolholder, which currents heat the sleeve section directly. The induction coil extends axially at least over the engagement length with which the tool shaft penetrates into the holding opening and ends with its winding axially approximately in the region of the front end on the tool side of the sleeve section. In the radial direction, the inner circumference of the induction coil runs at a distance from the sleeve section, in order to be able to use one and the same induction coil in the case of toolholders with a different external diameter of the sleeve section.
At its ends and on its outer circumference, the winding of the induction coil is encased with a jacket of magnetizable, that is to say a ferromagnetic or ferromagnetic, material whose high magnetic conductivity, as based on air, concentrates the magnetic flux substantially onto the jacket. The region of the jacket which is adjacent to the tool-side end of the sleeve section is formed as a substantially disk-like ring element, which rests with its inner circumference on the tool-side front end of the sleeve section and extends radially over the end surface of the winding of the induction coil as far as the outer circumference of the latter. The ring element forms a pole shoe which is intended to concentrate the magnetic flux of the induction coil onto the sleeve section.
It is an object of the invention to provide a device for the inductive heating of a toolholder which holds the tool with a shrink fit which, using simple means, permits the tool to be unclamped reliably from the toolholder.
The invention is based on a device for the inductive heating of a sleeve section of a toolholder which contains a central holding opening for a shaft of a rotary tool, said toolholder holding the shaft of the tool seated in the holding opening with a press fit and releasing it when heated. Such a device comprises
an induction coil, which encloses the sleeve section of the toolholder annularly with a radial spacing for the heating,
a generator that feeds the induction coil with electric current of periodically changing amplitude, and
a ring element, enclosing the shaft of the tool close to the tool-side end of the sleeve section of the toolholder and made of a magnetizable material that concentrates the magnetic flux, the ring element, in the region of its smallest diameter, being closely adjacent to the tool-side end of the sleeve section, in particular bearing on the sleeve section.
The object specified above is achieved, under a first aspect of the invention, in that the surface of the ring element which faces the interior of the induction coil axially, at least in a subregion, extends radially between the outer circumference of the tool-side end of the sleeve section and the inner circumference of the induction coil and axially obliquely radially outward away from the end of the sleeve section.
The magnetic flux generated by the induction coil must heat the sleeve section as uniformly as possible over the entire axial length used for clamping the tool shaft. The tool-side end of the sleeve section has proven to be critical, since the adjacent axial end of the induction coil in the case of conventional induction coils must not project or project only insignificantly beyond the axial end of the sleeve section, in order to avoid the concomitant heating of the tool shaft through the eddy currents induced in the tool shaft. Attempts have already been made in a known way (DE 199 15 412 A1) to concentrate the magnetic flux in the region of the tool-side end of the sleeve section by means of a pole shoe in the form of a flat disk of magnetizable material which tapers toward the tool shaft. In the case of this known pole shoe disk, the disk surface that faces the interior of the coil runs axially normally to the axis of rotation of the toolholder. It has been shown that the shielding effect, on the one hand, and the magnetic flux concentrating effect, on the other hand, can be improved if the surface that faces the interior of the coil of the ring element consisting of magnetizable material extends obliquely or conically radially outward away from the axial end of the sleeve section. Such a course of the material approaches the course of the flux lines which the induction coil would have in the air medium and facilitates the concentration of the magnetic flux in the magnetizable material of the ring element. The improved concentrating effect is accompanied by an improvement in the shielding effect. Shielding ring elements of the type explained above are advantageous in particular when the ring element, in its radial outer regions, adjoins the part of the magnetic circuit of the induction coil which runs in air, that is to say assumes the course of the flux lines of such a magnetic field circuit, specifically even when the magnetic flux from the induction coil, in its further course, runs in a single-part or multi-part yoke shell of magnetizable material which covers the induction coil on its outer circumference and at least one of its two axial ends.
Under a second aspect of the invention, provision is made for the ring element to be formed as a shielding collar which projects axially beyond the tool-side end of the sleeve section over an axial height of at least 1.5 times the diameter of the shaft of the tool and whose greatest diameter is smaller than the greatest winding diameter of the induction coil. Such a ring element, formed as a shielding collar, not only shields the tool shaft magnetically in the region of the tool-side end of the sleeve section but, on account of its considerable height, as based on the tool shaft diameter, is also able to concentrate magnetic flux running obliquely with respect to the axis of rotation. It goes without saying that both aspects of the invention can be embodied in one and the same ring element.
To the extent that magnetizable material is mentioned above, this is to be understood to include material with high magnetic conductivity, such as ferromagnetic material, such as soft iron laminate, or ferromagnetic material, for example ferrite or the like. The generator which feeds the induction coil can be an alternating current generator, but also a generator which outputs pulsed direct current. Generators for higher-frequency or high-frequency currents are preferred.
In a preferred configuration, the ring element not only has, on its side facing the interior of the induction coil, an outer circumferential surface which widens conically axially away from the toolholder but also, on its side facing axially away from the toolholder, an inner circumferential surface of this type which widens conically. The inner conical circumferential surface not only facilitates the insertion or removal of the tool but ensures a concentration of the magnetic flux at a distance from the tool shaft. The inner and the outer circumferential surfaces expediently have approximately the same cone angle.
In the region of its outer circumference, the ring element can have a substantially cylindrical circumferential surface which, in particular, can be formed by an annular extension which projects from the ring element in the region of its outer circumference, axially away from the toolholder. Such a circumferential surface improves the flux concentration behavior of the ring element, in particular when its greatest diameter is smaller than the smallest winding diameter of the induction coil and, in this case, the ring element concentrates magnetic flux which reaches the ring element in an air portion of the magnetic circuit.
The surface of the ring element which faces the interior of the induction coil axially and runs obliquely expediently runs inclined at an angle of between 10xc2x0 and 80xc2x0, preferably between 20xc2x0 and 70xc2x0, with respect to the axis of the shaft of the tool. The axial height of the ring element is preferably at least twice the shaft diameter of the tool.
The ring element can extend until close to the external winding diameter of the induction coil, but expediently has a maximum diameter which is smaller than the smallest winding diameter of the induction coil. Otherwise, in the region of its outer circumference and/or at one or both of its ends, the induction coil can be provided with a jacket of a magnetizable material that concentrates the magnetic flux, which ensures concentration of the magnetic flux outside the ring element and can also deflect the magnetic flux toward the ring element.
The internal diameter of the induction coil is selected to be slightly larger than the external diameter of the sleeve section of the toolholder, in order to be able to use one and the same induction coil in toolholders with different sleeve section diameters. In order to be able to match the ring element better to different shaft diameters of the tool in such a case, the ring element is preferably held on a structural unit which surrounds the induction coil, by means of a disk of non-magnetizable material, in particular of plastic or ceramic, to be specific expediently such that it can be replaced during operation.